Structural composite parts of aircraft designed with honeycomb core for stiffening and joggled flanges (such as ribs, spars, elevators, rudders, flaps, etc.) frequently experience producibility problems associated with these two design elements. Honeycomb core in composite parts can experience “core crush” which is a non-repairable defect that occurs when honeycomb core sections collapse. Core crush is thought to be related to the properties of the prepreg and woven composite materials. Composite prepreg materials contain a fiber reinforcement form (usually tape or fabric) that has been preimpregnated with a liquid resin and thermally advanced to a viscous stage. Composite woven materials contain interlaced yarns or fibers, usually in a planar structure, that establish a weave pattern from the yarns which is used as the fibrous constituent in an advanced composite lamina.
Parts with joggled flanges are also sensitive to porosity in the joggle region due to the inability of the prepreg to stay “seated” against the radius, and the joggle of the tool during lay-up and cure. Porosity is a defect involving unfilled space inside a material that frequently limits the material strength.
These core crush and porosity defects are producibility problems that are currently experienced worldwide. Core crush and porosity are the two predominant types of defects leading to part rejections in prepreg and woven composite materials since these conditions can be rarely be repaired.
Extensive research and development has been performed over the years by composite part fabricators in an effort to solve the core crush producibility problem. Core details and adjacent prepreg plies are stabilized in current production parts by various different methods (ply tie-downs, precured adhesive over the core, etc.) to reduce this core crush problem. Specific stabilization methods are documented in The Boeing Company's composite BAC Process Specifications which are incorporated herein by reference. However, these stabilization methods are unsatisfactory in that they are time consuming and add significant expense to the current production of sandwiched structure parts.
Likewise, extensive research and development has been performed in an attempt to address the porosity producibility problem in joggled parts. Particularly those parts utilizing the Boeing BMS 8-256 prepreg material (as described in the Boeing Materials Specification incorporated herein by reference). The extremely low flow properties of this prepreg's resin have particularly exacerbated the problem of porosity in parts designed with joggles. The BMS 8-256 prepreg material is currently one of the most widely used prepreg materials for composite secondary and primary structures for aircraft. Both material and process improvements have been evaluated in an effort to eliminate porosity. These have included the use of elastomeric pressure pads against the joggle during cure, decreasing part staging time prior to the cure, increasing the tack and drape of the prepreg, etc. These measures have yet to totally and reliably eliminate porosity in the joggles of parts fabricated with a prepreg material having low flow resin properties.
There is a continuing need in the art for a structural composite material designed with a honeycomb core that is resistant to core crush and porosity defects, particularly for a material having high resin viscosity and/or low flow properties.